1. Field of the Invention
This invention relates to a loose draw detector for a paper machine wet press. Such structure of this type, generally, use laser triangulation displacement sensors mounted and positioned to measure the point of web release from the center roll and measure paper web wrinkles in the form of rapid fluctuations/decreases in the standoff distance between sensors and the web passing over the leadout roll to determine loose draw.
2. Description of the Related Art
The tension of a dry paper web can be conventionally measured via several approaches, but the press section of the web is still wet (water content by weight is at least 50%) and has very little tensile strength, in that any direct measurement of tension would compromise web transfer. Some paper machines have an xe2x80x9copen drawxe2x80x9d between rolls where the unsupported sheet run is horizontally oriented. In this situation, machine drive speeds can be regulated to maintain the web vertical location at a desired point.
It is known to attempt to reduce the incidence of draw-related web breaks at the press section of the paper machine. Exemplary of such prior art is U.S. Pat. No. 5,002,638 (""638) to T. G. Gulya et al., entitled xe2x80x9cPapermaking Machine in Which the Paper Web is Supported in the Draw Between the Press and Dryer Sections.xe2x80x9d The ""638 reference employs a press configuration that eliminates an xe2x80x9copen drawxe2x80x9d between the last press nip and subsequent steam drying section, in that the press felt is made to wrap the first dryer can so that the felt conveys the sheet into the dryer section.
It is also know to employ a device which is intended to reduce web flutter without making adjustment to machine drive speed. Exemplary of such prior art is U.S. Pat. No. 5,094,718 (""718) to W. H. Friend, entitled xe2x80x9cMethod and Apparatus for Control of Web Flutter.xe2x80x9d The ""718 measures acoustic disturbances set up when the web flutter is present, then actively modulates/reduces the amount of flutter. The latter is accomplished by broadcasting a second acoustic disturbance, whose phase is shifted approximately 180 degrees from that of the measured disturbance, so that the two cancel each other. However, a more advantageous device would be one which employs a means to keep control to a minimum level the amount of web draw at the first point in a paper machine where the web is unsupported, namely between two successive press nips.
It is apparent from the above that there exists a need in the art for a loose draw detector for a paper machine wet press which measures the point of web release from the center roll and measure wrinkles in the form of rapid fluctuations/decreases in the standoff distance between the sensor and the web passing over leadout roll. It is the purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.
Generally speaking, this invention fulfills these needs by providing a paper web loose draw detection system for a paper machine wet press, comprising a paper machine wet press section including a center roll and a paper leadout roll located substantially adjacent to the center roll, a paper web which is traversed through the wet press section such that the web contacts the center roll then the leadout roll, and a first loose draw detector means located substantially between the center roll and the leadout roll and a second loose draw detector means located substantially adjacent to the leadout roll, such that a loose draw in the web is substantially detected by the first and second detection means by measuring a position of the web through a beam of light from the first and second detection means, wherein the beam of light from the first detector means is substantially tangent to the center roll so as to substantially coincide with a region in which the web separates from the center roll and wherein the beam of light from the second detector means is substantially perpendicular to the web as the web passes over the paper leadout roll.
In certain preferred embodiments, the first and second detection means are laser triangulation sensors.
In another further preferred embodiment, since the problem at hand, namely the source of web breaks, is a difficulty of web release from the press roll, this loose draw detector configuration effectively monitors the variable of concern for draw-related breaks in a press section.
The preferred detector, according to this invention, offers the following advantages: lightness in weight; good stability; excellent durability; loose draw detection; ease of measurement of web release; ease of measurement of wrinkles; and good economy. In fact, in many of the preferred embodiments, these factors of durability, loose draw detection, measurement of web release, and measurement of wrinkles are optimized to the extent that is considerably higher than heretofore achieved in prior, known detectors.
The above and other features of the present invention, which will become more apparent as the description proceeds, are best understood by considering the following detailed description in conjunction with the accompany FIGURE, wherein: